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#11
A program like UltraISO will allow you to do this. I guess I was making the assumption that those following the guide would have software like this, since most of us are dealing with the Windows 7 discs in the form of .isos already.
A trick many have used to end up with a bootable .iso file is to use UltraISO. Once you have your contents done following my link above, you then open one of the original, bootable .iso files. You delete the contents of the disc, and add in the new folder you created to the structure looks the same. It's very easy, as UltraISO resembles Windows Explorer. Then you save it as a file called Windows 7 Combined.iso or something like that.
You'll be left with a bootable disc image that has both platforms. The Microsoft tool I linked to will take that file and turn it into a bootable flash drive, as most of us do for quicker installs.